1. Field of the Invention
The present invention relates to a polishing apparatus and a polishing method for polishing a substrate, such as a wafer.
2. Description of the Related Art
With a recent trend toward higher integration and higher density in semiconductor devices, circuit interconnects become finer and finer and the number of levels in multilayer interconnect is increasing. In the fabrication process of the multilayer interconnect with finer circuit, as the number of interconnect levels increases, film coverage (or step coverage) of step geometry is lowered in thin firm formation because surface steps grow while following surface irregularities on a lower layer. Therefore, in order to fabricate the multilayer interconnect, it is necessary to improve the step coverage and planarize the surface. It is also necessary to planarize semiconductor device surfaces so that irregularity steps formed thereon fall within a depth of focus in optical lithography. This is because finer optical lithography entails shallower depth of focus.
Accordingly, the planarization of the semiconductor device surfaces is becoming more important in the fabrication process of the semiconductor devices. Chemical mechanical polishing (CMP) is the most important technique in the surface planarization. This chemical mechanical polishing is a process of polishing a wafer with use of a polishing apparatus by placing the wafer in sliding contact with a polishing surface of a polishing pad while supplying a polishing liquid containing abrasive grains, such as silica (SiO2), onto the polishing surface.
The polishing apparatus of this type has a polishing table that supports the polishing pad thereon, and a substrate holder for holding the wafer. The substrate holder is often called a top ring or a polishing head. This polishing apparatus polishes the wafer as follows. The substrate holder holds the wafer and presses it against the polishing surface of the polishing pad at predetermined pressure. The polishing table and the substrate holder are moved relative to each other to bring the wafer into sliding contact with the polishing surface to thereby polish a surface of the wafer.
When polishing the wafer, if a relative pressing force applied between the wafer and the polishing pad is not uniform over the surface of the wafer in its entirety, lack of polishing or excessive polishing would occur depending on the pressing force applied to each portion of the water. Thus, in order to even the pressing force exerted on the wafer, the substrate holder has at its lower part a pressure chamber formed by a flexible membrane. This pressure chamber is supplied with fluid, such as air, to press the wafer through the flexible membrane under the fluid pressure.
However, because the above-described polishing pad has elasticity, the pressing force becomes non-uniform in an edge portion (a peripheral portion) of the wafer during polishing of the water. Such non-uniform pressing force would result in so-called “rounded edge” which is excessive polishing that occurs only in the edge portion of the wafer. In order to prevent such rounded edge, the substrate holder has a retaining ring for retaining the edge portion of the wafer. This retaining ring is configured to be vertically movable relative to a top ring body (or carrier head body) and press a region in the polishing surface of the polishing pad around the wafer.
As the types of semiconductor devices have been increasing dramatically in recent years, there is an increasing demand for controlling a polishing profile in the wafer edge portion for each device or each C process (e.g., an oxide film polishing process and a metal film polishing process). One of the reasons is that each wafer has a different initial film-thickness distribution because a film-forming process, which is performed prior to the CMP process, varies depending on the type of film. Typically, a wafer is required to have a uniform film-thickness distribution over its entire surface after the CMP process. Therefore, different initial film-thickness distributions necessitate different polishing profiles.
Other reason is that types of polishing pads and polishing liquids, both of which are consumables in the polishing apparatus, are increasing greatly from a viewpoint of costs. Use of different polishing pads or different polishing liquids results in different polishing profiles particularly in the wafer edge portion. Because the polishing profile in the wafer edge portion can greatly affect a product yield, it is very important to precisely control the polishing profile in the wafer edge portion.
As described above, for the purpose of preventing the rounded edge of the wafer, the conventional substrate holder has the retaining ring configured to press the polishing surface of the polishing pad around the wafer. It is possible to control a polishing rate in the wafer edge portion by regulating pressure of the retaining ring. However, changing the pressure of the retaining ring could result in a change in the polishing rate not only in the wafer edge portion, but also in other regions with a relatively large area. Therefore, this approach is not suitable in the case of precisely controlling the polishing profile in the wafer edge portion.